Steel material for line pipes, method for producing the same, line pipe, and method for producing the line pipe

ABSTRACT

A steel material for line pipes has a specific composition. The metallic microstructure of the steel material at a ⅛-plate thickness position below the surface includes bainite of an area fraction of 85% or more, polygonal ferrite of an area fraction of 10% or less, and martensite-austenite constituent of an area fraction of 5% or less. The 0.23% compressive strength of a portion of the steel material which extends from the surface to the ⅛-plate thickness position in a transverse direction is 340 MPa or more. The temperature at which a percent ductile fracture of the steel material measured in a DWTT test becomes 85% or more is −10° C. or less.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2020/012169, filedMar. 19, 2020, which claims priority to Japanese Patent Application No.2019-062703, filed Mar. 28, 2019, the disclosures of these applicationsbeing incorporated herein by reference in their entireties for allpurposes.

FIELD OF THE INVENTION

The present invention relates to a steel material for line pipes, amethod for producing the steel material for line pipes, a line pipe, anda method for producing the line pipe. The present invention relates to asteel material for line pipes which is suitable as a material for linepipes used for the transportation of oil and natural gas and isparticularly suitable as a material for offshore pipelines, which arerequired to have a high collapse resistant performance, a method forproducing the steel material for line pipes, such a line pipe, and amethod for producing the line pipe. The term “compressive strength” usedherein refers to 0.23% compressive proof strength, unless otherwisespecified, and is also referred to as “compressive yield strength”.

BACKGROUND OF THE INVENTION

With an increasing demand for energy, the development of oil and naturalgas pipelines has been active. Various pipelines that extend across seahave been developed in order to cope with a situation where gas fieldsor oil fields are located at remoter places or versatility in transportroutes. Line pipes used as offshore pipelines have a larger wallthickness than onshore pipelines in order to prevent collapse due towater pressure, and are required to have a high degree of roundness. Inaddition, as for the properties of offshore line pipes, the offshoreline pipes need to have a high compressive strength in order to resistthe compressive stress caused by external water pressure in thecircumferential direction of the pipes.

Since the final step of a method for making UOE steel pipes includes apipe expanding process, and after the pipes have been subjected to atensile deformation in the circumferential direction of the pipes, thesteel pipes are constructed on the sea bed and compressed by externalwater pressure in the circumferential direction of the pipes.Consequently, there is a problem that compressive yield strength will bereduced disadvantageously due to the Bauschinger effect.

There have been various studies of improvement of the collapse resistantperformance of UOE steel pipes. Patent Literature 1 discloses a methodin which a steel pipe is heated by Joule heating to expand the pipe andthe temperature is subsequently held for a certain period of time ormore.

As a method in which heating is performed subsequent to the pipeexpansion as described above in order to restore from the reduction incompressive yield strength caused by the Bauschinger effect, PatentLiterature 2 proposes a method in which the outer surface of a steelpipe is heated to a temperature higher than that of the inner surface inorder to restore from the impact due to the Bauschinger effect caused inthe outer surface-side portion of the steel pipe which has beensubjected to a tensile deformation and to maintain the strain hardeningof the inner surface-side portion due to compression. Patent Literature3 proposes a method in which, in a steel plate making process using asteel containing Nb and Ti, accelerated cooling is performed from atemperature equal to or greater than the Ar₃ transformation temperatureto 300° C. or less subsequent to hot rolling and heating is performedafter a steel pipe has been formed by the UOE process.

On the other hand, as a method in which the compressive strength of asteel pipe is increased by adjusting the conditions under which thesteel pipe is formed, instead of performing heating subsequent to thepipe expansion, Patent Literature 4 discloses a method in which thecompression rate at which compression is performed when a steel pipe isformed using the O-ing press is set to be higher than the expansionratio at which pipe expansion is performed in the subsequent step.

Patent Literature 5 discloses a method in which the diameter of a steelpipe which passes through the vicinity of a weld zone, which has a lowercompressive strength, and the position that forms an angle of 180° withrespect to the weld zone is set to be the maximum diameter of the steelpipe in order to enhance the collapse resistant performance of the steelpipe.

Patent Literature 6 proposes a steel plate capable of limiting areduction in yield stress due to the Bauschinger effect, which isproduced by performing reheating subsequent to accelerated cooling toreduce the fraction of the hard second phase in the surface-layerportion of the steel plate.

Patent Literature 7 proposes a method for producing a high-strengthsteel plate for line pipes for sour gas service having a thickness of 30mm or more, in which the surface-layer portion of a steel plate isheated in a reheating process performed subsequent to acceleratedcooling while a rise in the temperature of the center of the steel plateis suppressed.

PATENT LITERATURE

PTL 1: Japanese Unexamined Patent Application Publication No. 9-49025

PTL 2: Japanese Unexamined Patent Application Publication No.2003-342639

PTL 3: Japanese Unexamined Patent Application Publication No. 2004-35925

PTL 4: Japanese Unexamined Patent Application Publication No.2002-102931

PTL 5: Japanese Unexamined Patent Application Publication No.2003-340519

PTL 6: Japanese Unexamined Patent Application Publication No. 2008-56962

PTL 7: Japanese Unexamined Patent Application Publication No. 2009-52137

SUMMARY OF THE INVENTION

According to the method described in Patent Literature 1, dislocationintroduced by the pipe expansion is recovered and, consequently,compressive strength is increased. However, this method requires theJoule heating to be continued for five minutes or more subsequent to thepipe expansion and is therefore poor in terms of productivity.

In the method described in Patent Literature 2, it is necessary toindividually manage the temperatures at which the outer and innersurfaces of a steel pipe are heated and the amounts of time during whichthe outer and inner surfaces of the steel pipe are heated. This isdifficult in terms of the actual manufacture. It is considerablydifficult to manage the quality of steel pipes in a mass productionprocess. The method described in Patent Literature 3 requires theaccelerated cooling stop temperature in the production of a steel plateto be a low temperature of 300° C. or less. This increases thedistortion of a steel plate and degrades the roundness of a steel pipeproduced by the UOE process. Furthermore, since the accelerated coolingis performed from a temperature of the Ar_(a) temperature or more, it isnecessary to perform rolling at a relatively high temperature. This willresult in the degradation of toughness.

According to the method described in Patent Literature 4, tensilepre-strain substantially does not occur in the circumferential directionof the pipe. Accordingly, the Bauschinger effect is not produced and ahigh compressive strength can be achieved. However, a low expansionratio makes it difficult to maintain the roundness of a steel pipe andmay degrade the collapse resistant performance of the steel pipe.

The portion of a pipeline which is required to have certain collapseresistant performance when the pipeline is actually constructed is aportion (sag-bend portion) subjected to a bending deformation when thepipe reaches the sea bed. When a pipeline is constructed on the sea bed,girth welding is performed without reference to the positions of weldzones of steel pipes. Therefore, even if steel pipes are produced byperforming pipe forming and welding such that a cross section of each ofthe steel pipes has the maximum diameter at the seam weld zone asdescribed in Patent Literature 5, it is not possible to determine thepositions of the seam weld zones when a pipeline is actuallyconstructed. Thus, the technology according to Patent Literature 5 doesnot produce any advantageous effects in reality.

The steel plate described in Patent Literature 6 needs to be heated inthe reheating step until the center of the steel plate is heated. Thismay result in the degradation of a DWTT (drop weight tear test)property. Therefore, it is difficult to use this steel plate forproducing deep-ocean thick-walled line pipes. In addition, the steelplate has room for improvement in terms of increase in the thickness ofthe steel plate. Moreover, the collapse resistant performance of a steelpipe is in correlation with the compression flow stress which is closeto the elastic limit and acts in the inner surface layer of the pipe. InPatent Literature 6, collapse resistant performance is determined at a¼-plate thickness position. However, even when a steel pipe has a highcompressive strength at a ¼-plate thickness position, the actualadvantageous effect on the critical collapse pressure of the steel pipeis small.

According to the method described in Patent Literature 7, the fractionof the hard second phase in the surface-layer portion of a steel platecan be reduced while the degradation of a DWTT (drop weight tear test)property is limited. This may reduce the hardness of a surface-layerportion and inconsistencies in the material quality of the steel plate.Furthermore, the reduction in the fraction of the hard second phase mayreduce the Bauschinger effect. However, in the technology described inPatent Literature 7, the surface of a steel plate is heated to 550° C.or more. This may reduce the compressive strength of the surface layerand consequently degrade collapse resistant performance.

Aspects of the present invention were made in view of theabove-described circumstances. An object according to aspects of thepresent invention is to provide a steel material for line pipes having aheavy wall thickness, a certain compressive strength required forapplying the steel material to offshore pipelines, excellentlow-temperature toughness, an excellent DWTT property, and excellentcollapse resistant performance, a method for producing the steelmaterial for line pipes, a line pipe having the required compressivestrength, excellent low-temperature toughness, an excellent DWTTproperty, and excellent collapse resistant performance, and a method forproducing the line pipe.

Note that, the expression “having excellent collapse resistantperformance” used herein means that, as for the steel material for linepipes, the 0.23% compressive strength of a portion of the steelmaterial, the portion extending from the surface of the steel materialto a ⅛-plate thickness position that is a position ⅛ of the thickness ofthe steel material below the surface, in the transverse direction(rolling orthogonal direction) is 340 MPa or more and, as for the linepipe, the 0.23% compressive strength of a portion of the line pipe, theportion extending from the inner surface of the line pipe to a ⅛-wallthickness position that is a position ⅛ of the wall thickness of theline pipe below the inner surface, in the circumferential direction at amajor axis position of the pipe is 340 MPa or more and the collapsepressure of the line pipe is 35 MPa or more.

The inventors of the present invention conducted extensive studies inorder to enhance collapse resistant performance and, as a result, foundthe following facts.

(a) The reduction in compressive strength due to the Bauschinger effectis induced by the back stress caused as a result of the accumulation ofdislocations at the interfaces between different phases and in the hardsecond phase. For preventing this, first, it is effective to form auniform microstructure in order to reduce the interfaces between thesoft and hard phases, at which dislocations accumulate. Accordingly,forming a metallic microstructure composed primarily of bainite in whichthe formation of soft polygonal ferrite and hard martensite-austeniteconstituent is suppressed results in limiting the reduction incompressive strength due to the Bauschinger effect.

(b) It is difficult to completely inhibit the formation of themartensite-austenite constituent (hereinafter, may be referred to simplyas “MA”) in high-strength steel produced by accelerated cooling and, inparticular, thick-walled steel plates used for producing offshorepipelines because such high-strength steel and thick-walled steel plateshave high hardenability as a result of containing large amounts ofalloying elements to achieve an intended strength. However, thereduction in compressive strength due to the Bauschinger effect can belimited when MA is decomposed into cementite by, for example,suppressing the formation of MA by chemical composition control orperforming reheating subsequent to accelerated cooling. Althoughperforming reheating more than necessary reduces compressive strength,the required compressive strength can be achieved by controlling thereheating temperature of the surface layer.

(c) While evaluation of compressive strength is commonly made on thebasis of 0.5% compressive strength, collapse resistant performance is incorrelation with 0.23% compressive strength, which is close to theelastic limit, of the inner surface layer of a line pipe. Thus,increasing the 0.23% compressive strength of a portion of a line pipewhich extends from the inner surface of the pipe to a position ⅛ of thewall thickness below the inner surface will enhance collapse resistantperformance.

Aspects of the present invention were made on the basis of the abovefindings and additional studies. The summary of aspects of the presentinvention is as follows.

[1] A steel material for line pipes, the steel material having acomposition containing, by mass, C: 0.030% to 0.10%, Si: 0.01% to 0.15%,Mn: 1.0% to 2.0%, Nb: 0.005% to 0.050%, Ti: 0.005% to 0.025%, and Al:0.08% or less, the composition further containing one or more elementsselected from, by mass, Cu: 0.5% or less, Ni: 1.0% or less, Cr: 1.0% orless, Mo: 0.5% or less, and V: 0.1% or less, wherein a Ceq valuerepresented by Formula (1) is 0.35 or more and a Pcm value representedby Formula (2) is 0.20 or less, with the balance being Fe and incidentalimpurities,

wherein a metallic microstructure of the steel material at a ⅛-platethickness position relative to a surface of the steel material includesbainite of an area fraction pf 85% or more, polygonal ferrite or an areafraction of 10% or less, and martensite-austenite constituent of an areafraction of 5% or less, and

wherein a 0.23% compressive strength of a portion of the steel material,the portion extending from the surface of the steel material to the⅛-plate thickness position, in a rolling orthogonal direction is 340 MPaor more, and a temperature at which a percent ductile fracture of thesteel material measured in a DWTT test becomes 85% or more is −10° C. orless,

Ceq value=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5   (1)

Pcm value=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10   (2)

where, in Formulae (1) and (2), the symbol of each element representsthe content (mass %) of the element and is zero when the steel materialdoes not contain the element.

[2] The steel material for line pipes described in [1], the steelmaterial further including, by mass, Ca: 0.0005% to 0.0035%.

[3] A method for producing a steel material for line pipes, wherein a0.23% compressive strength of a portion of the steel material, theportion extending from a surface of the steel material to a ⅛-platethickness position, in a rolling orthogonal direction is 340 MPa ormore, and a temperature at which a percent ductile fracture of the steelmaterial measured in a DWTT test becomes 85% or more is −10° C. or less,the method including:

heating a steel having the composition described in [1] or [2] to atemperature of 1000° C. to 1200° C.;

hot-rolling the steel such that a cumulative rolling reduction ratio ina non-recrystallization temperature range is 60% or more, and such thata finish rolling temperature is equal to or greater than an Ar₃transformation temperature and equal to or less than (Ar₃ transformationtemperature+60° C.);

subsequently performing accelerated cooling from a temperature equal toor greater than the Ar₃ transformation temperature to a temperature of200° C. to 450° C. at a cooling rate of 10° C./s or more; and

then performing reheating such that a temperature of the steel materialat the ⅛-plate thickness position is 350° C. or more and such that atemperature of the surface of the steel material is 530° C. or less.

[4] A line pipe having a composition containing, by mass, C: 0.030% to0.10%, Si: 0.01% to 0.15%, Mn: 1.0% to 2.0%, Nb: 0.005% to 0.050%, Ti:0.005% to 0.025%, and Al: 0.08% or less, the composition furthercontaining one or more elements selected from, by mass, Cu: 0.5% orless, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 0.5% or less, and V: 0.1%or less, wherein a Ceq value represented by Formula (1) is 0.35 or moreand a Pcm value represented by Formula (2) is 0.20 or less, with thebalance being Fe and incidental impurities,

wherein a metallic microstructure of the line pipe at a ⅛-wall thicknessposition relative to an inner surface of the line pipe includes bainiteof an area fraction of 85% or more, polygonal ferrite of an areafraction of 10% or less, and martensite-austenite constituent of an areafraction of 5% or less, and

wherein a 0.23% compressive strength of a portion of the line pipe, theportion extending from the inner surface of the line pipe to the ⅛-wallthickness position, in a circumferential direction of the line pipe at amajor axis position of the line pipe is 340 MPa or more, a collapsepressure of the line pipe is 35 MPa or more, and a temperature at whicha percent ductile fracture of the line pipe measured in a DWTT testbecomes 85% or more is −10° C. or less,

Ceq valu=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5   (1)

Pcm value=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10   (2)

where, in Formulae (1) and (2), the symbol of each element representsthe content (mass%) of the element and is zero when the line pipe doesnot contain the element.

[5] The line pipe described in [4], the line pipe further including, bymass, Ca: 0.0005% to 0.0035%.

[6] The line pipe described in [4] or [5], the line pipe furtherincluding a coating layer.

[7] A method for producing a line pipe, wherein a 0.23% compressivestrength of a portion of the line pipe, the portion extending from aninner surface of the line pipe to a ⅛-wall thickness position, in acircumferential direction of the line pipe at a major axis position ofthe line pipe is 340 MPa or more, a collapse pressure of the line pipeis 35 MPa or more, and a temperature at which a percent ductile fractureof the line pipe measured in a DWTT test becomes 85% or more is −10° C.or less, the method including cold forming the steel material for linepipes described in [1] or [2] into a pipe-like shape; joining buttingedges to each other by seam welding; and subsequently performing pipeexpansion at an expansion ratio of 1.2% or less to produce a pipe.

[8] A method for producing a line pipe, wherein a 0.23% compressivestrength of a portion of the line pipe, the portion extending from aninner surface of the line pipe to a ⅛-wall thickness position, in acircumferential direction of the line pipe at a major axis position ofthe line pipe is 340 MPa or more, a collapse pressure of the line pipeis 35 MPa or more, and a temperature at which a percent ductile fractureof the line pipe measured in a DWTT test becomes 85% or more is −10° C.or less, the method including cold forming a steel material for linepipes produced by the method described in [3] into a pipe-like shape;joining butting edges to each other by seam welding; and subsequentlyperforming pipe expansion at an expansion ratio of 1.2% or less toproduce a pipe.

[9] The method for producing a line pipe described in [7] or [8], themethod further including performing a coating treatment subsequent tothe pipe expansion, the coating treatment including heating the pipesuch that a temperature of the surface of the pipe reaches 200° C. ormore.

According to aspects of the present invention, a steel material for linepipes which has excellent collapse resistant performance can beproduced. Aspects of the present invention can be suitably applied todeep-ocean pipelines.

According to aspects of the present invention, a thick-walled line pipehaving excellent low-temperature toughness and a high compressivestrength can be provided without applying special conditions for formingsteel pipes or performing a heat treatment subsequent to pipeproduction.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

An embodiment of the present invention is described below. Whenreferring to the contents of constituent elements, the symbol “%” refersto “% by mass” unless otherwise specified.

1. Chemical Composition of Steel Material for Line Pipes or Line Pipe

C: 0.030% to 0.10%

C is an element most effective in increasing the strength of a steelplate produced by accelerated cooling. However, if the C content is lessthan 0.030%, a sufficiently high strength may fail to be maintained.Accordingly, the C content is limited to 0.030% or more and ispreferably 0.040% or more. On the other hand, if the C content is morethan 0.10%, toughness becomes degraded. In addition, the formation of MAmay be accelerated. This results in a reduction in compressive strength.Accordingly, the C content is limited to 0.10% or less and is preferably0.098% or less.

Si: 0.01% to 0.15%

Si is used for deoxidization. However, if the Si content is less than0.01%, a sufficient deoxidation effect may fail to be achieved.Accordingly, the Si content is limited to 0.01% or more and ispreferably 0.03% or more. On the other hand, if the Si content is morethan 0.15%, toughness becomes degraded. In addition, the formation of MAmay be accelerated. This results in a reduction in compressive strength.

Accordingly, the Si content is limited to 0.15% or less and ispreferably 0.10% or less.

Mn: 1.0% to 2.0%

The Mn content is limited to 1.0% to 2.0%. Mn is used for increasingstrength and enhancing toughness. However, if the Mn content is lessthan 1.0%, the above advantageous effects may fail to be produced to asufficient degree. Accordingly, the Mn content is limited to 1.0% ormore and is preferably 1.5% or more. On the other hand, if the Mncontent is more than 2.0%, toughness may become degraded. Accordingly,the Mn content is limited to 2.0% or less and is preferably 1.95% orless.

Nb: 0.005% to 0.050%

Nb reduces the size of microstructures and thereby enhances toughness.Nb also causes the formation of carbides, which increase strength.However, if the Nb content is less than 0.005%, the above advantageouseffects may fail to be produced to a sufficient degree. Accordingly, theNb content is limited to 0.005% or more and is preferably 0.010% ormore. On the other hand, if the Nb content is more than 0.050%, thetoughness of a weld heat-affected zone (HAZ) caused by welding maybecome degraded. Accordingly, the Nb content is limited to 0.050% orless and is preferably 0.040% or less.

Ti: 0.005% to 0.025%

Ti reduces the likelihood of austenite grains coarsening during heatingof slabs by the pinning effect of TiN and thereby enhances toughness.However, if the Ti content is less than 0.005%, the above advantageouseffects may fail to be produced to a sufficient degree. Accordingly, theTi content is limited to 0.005% or more and is preferably 0.008% ormore. On the other hand, if the Ti content is more than 0.025%,toughness may become degraded. Accordingly, the Ti content is limited to0.025% or less and is preferably 0.023% or less.

Al: 0.08% or Less

Al is used as a deoxidizing agent. In order to produce the advantageouseffect, the Al content is preferably 0.01% or more. However, if the Alcontent is more than 0.08%, the cleanliness of steel may become degradedand toughness may become degraded. Accordingly, the Al content islimited to 0.08% or less. The Al content is preferably 0.05% or less.

In accordance with aspects of the present invention, one or moreelements selected from Cu: 0.5% or less, Ni: 1.0% or less, Cr: 1.0% orless, Mo: 0.5% or less, and V: 0.1% or less are contained.

Cu: 0.5% or Less

Cu is an element effective in improving toughness and increasingstrength. However, if the Cu content is more than 0.5%, the HAZtoughness of a weld zone becomes degraded. Accordingly, in the casewhere Cu is used, the Cu content is limited to 0.5% or less. The lowerlimit for the Cu content is not specified. In the case where Cu is used,the Cu content is preferably 0.01% or more.

Ni: 1.0% or Less

Ni is an element effective in improving toughness and increasingstrength. However, if the Ni content is more than 1.0%, the HAZtoughness of a weld zone may become degraded. Accordingly, in the casewhere Ni is used, the Ni content is limited to 1.0% or less. The lowerlimit for the Ni content is not specified. In the case where Ni is used,the Ni content is preferably 0.01% or more.

Cr: 1.0% or Less

Cr is an element that enhances hardenability and thereby effectivelyincreases strength. However, if the Cr content is more than 1.0%, theHAZ toughness of a weld zone becomes degraded. Accordingly, in the casewhere Cr is used, the Cr content is limited to 1.0% or less. The lowerlimit for the Cr content is not specified. In the case where Cr is used,the Cr content is preferably 0.01% or more.

Mo: 0.5% or Less

Mo is an element effective in improving toughness and increasingstrength. However, if the Mo content is more than 0.5%, the HAZtoughness of a weld zone may become degraded. Accordingly, in the casewhere Mo is used, the Mo content is limited to 0.5% or less. The lowerlimit for the Mo content is not specified. In the case where Mo is used,the Mo content is preferably 0.01% or more.

V: 0.1% or Less

V is an element that forms complex carbides as well as Nb and Ti and ismarkedly effective in increasing strength by precipitationstrengthening. However, if the V content is more than 0.1%, the HAZtoughness of a weld zone may become degraded. Accordingly, in the casewhere V is used, the V content is limited to 0.1% or less. The lowerlimit for the V content is not specified. In the case where V is used,the V content is preferably 0.01% or more.

In accordance with aspects of the present invention, the Ceq valuerepresented by Formula (1) is 0.35 or more and the Pcm value representedby Formula (2) is 0.20 or less.

Ceq Value: 0.35 or More

The Ceq value is limited to 0.35 or more. The Ceq value is representedby Formula (1) below. The Ceq value is in correlation with the strengthof base metal and is used as a measure of strength. If the Ceq value isless than 0.35, a high tensile strength of 570 MPa or more may fail tobe achieved. Accordingly, the Ceq value is limited to 0.35 or more. TheCeq value is preferably 0.36 or more.

Ceq value=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5   (1)

In Formula (1), the symbol of each element represents the content(mass%) of the element and is zero when the chemical composition doesnot contain the element.

Pcm Value: 0.20 or Less

The Pcm value is limited to 0.20 or less. The Pcm value is representedby Formula (2) below. The Pcm value is used as a measure of weldability;the higher the Pcm value, the lower the toughness of a weld HAZ. The Pcmvalue needs to be strictly limited particularly in a thick-walledhigh-strength steel, because the impact of the Pcm value is significantin a thick-walled high-strength steel. Accordingly, the Pcm value islimited to 0.20 or less. The Pcm value is preferably 0.19 or less.

Pcm value=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10   (2)

In Formula (2), the symbol of each element represents the content (mass%) of the element and is zero when the composition does not contain theelement.

In accordance with aspects of the present invention, the chemicalcomposition may contain the following element as needed.

Ca: 0.0005% to 0.0035%

Ca is an element effective in controlling the shape of sulfideinclusions and improve ductility. The advantageous effects are producedwhen the Ca content is 0.0005% or more. Accordingly, in the case wherethe chemical composition contains Ca, the Ca content is preferably0.0005% or more. When the Ca content exceeds 0.0035%, the advantageouseffects peak out; on the contrary, cleanliness may become degraded and,consequently, toughness may become degraded. Thus, in the case where thechemical composition contains Ca, the Ca content is preferably 0.0035%or less.

The remaining part of the chemical composition which is other than theabove-described elements, that is, the balance, includes Fe andincidental impurities. The chemical composition may contain an elementother than the above-described elements such that the action andadvantageous effects according to aspects of the present invention arenot impaired.

2. Metallic Microstructure of Steel Material for Line Pipes or Line Pipe

In accordance with aspects of the present invention, the metallicmicrostructure of the steel material at a ⅛-plate thickness positionthat is a position ⅛ of the thickness of the steel material below thesurface or the metallic microstructure of the line pipe at a ⅛-wallthickness position that is a position ⅛ of the wall thickness of thepipe below the inner surface of the line pipe is specified. Inaccordance with aspects of the present invention, controlling themetallic microstructure of the steel material at the position ⅛ of thethickness of the steel material below the surface increases thecompressive strength of the steel material. As a result, a steelmaterial for line pipes or line pipe having excellent collapse resistantperformance can be produced.

Area Fraction of Bainite Is 85% or More

The metallic microstructure according to aspects of the presentinvention is composed primarily of bainite in order to limit thereduction in compressive strength due to the Bauschinger effect. Theexpression “the metallic microstructure according to aspects of thepresent invention is composed primarily of bainite” means that the areafraction of bainite in the entire metallic microstructure is 85% ormore. For limiting the reduction in compressive strength due to theBauschinger effect, the metallic microstructure is desirably composedonly of bainite in order to prevent the accumulation of dislocations atthe interfaces between different phases and in the hard second phase.However, when the fraction of the balance of microstructures other thanbainite is 15% or less, they may be acceptable.

Area Fractions of Polygonal Ferrite and Martensite-Austenite ConstituentAre 10% or Less and 5% or Less, Respectively

For reducing the Bauschinger effect and achieving a high compressivestrength, it is desirable to form a uniform microstructure free of softpolygonal ferrite phase or hard martensite-austenite constituent inorder to reduce the likelihood of dislocations locally accumulating inthe microstructure during deformation. Accordingly, in addition toforming a microstructure composed primarily of bainite as describedabove, the area fractions of polygonal ferrite and themartensite-austenite constituent are limited to 10% or less and 5% orless, respectively. The area fraction of the martensite-austeniteconstituent may be 0%. The area fraction of polygonal ferrite may be 0%.

The metallic microstructure according to aspects of the presentinvention may include any phases other than bainite, polygonal ferrite,or the martensite-austenite constituent as long as it includes theabove-described structure. Examples of the other phases includepearlite, cementite, and martensite. The amount of the other phases ispreferably minimized; the area fraction of the other phases at aposition ⅛ of the plate thickness below the surface of the steelmaterial is preferably 5% or less.

In accordance with aspects of the present invention, the metallicmicrostructure of a portion of the steel material that extends from theposition ⅛ of the plate thickness below the surface of the steelmaterial toward the center of the steel material in the plate-thicknessdirection or a portion of the line pipe that extends from the position ⅛of the wall thickness below the inner surface of the pipe toward thecenter of the pipe in the wall-thickness direction is not limited.However, the fraction of bainite in the above metallic microstructure ispreferably 70% or more and is more preferably 75% or more inconsideration of the balance between properties such as strength andtoughness. Examples of the balance of microstructure include ferrite,pearlite, martensite, and martensite-austenite constituent (MA). Whenthe fraction of the above balance of microstructures is 30% or less andis more preferably 25% or less in total, they may be acceptable.

In accordance with aspects of the present invention, when the metallicmicrostructure of the steel material at the position ⅛ of the thicknessof the steel material below the surface satisfies the above conditions,the compressive strength of a portion of the steel material whichextends from the surface to the ⅛-plate thickness position and thecompressive strength of a portion of the line pipe which extends fromthe inner surface to the ⅛-wall thickness position can be increased and,consequently, excellent collapse resistant performance can be achieved.

3. Method for Producing Steel Material for Line Pipes

The method for producing a steel material for line pipes according toaspects of the present invention includes heating a steel slab havingthe above-described chemical composition, hot rolling the steel slab,subsequently performing accelerated cooling, and then performingtempering (reheating). The reasons for limiting the productionconditions are described below. Hereinafter, the term “temperature”refers to the average temperature of the steel material (steel plate) inthe thickness direction, unless otherwise specified. The averagetemperature of the steel plate in the thickness direction is determinedon the basis of thickness, surface temperature, cooling conditions, etc.by simulation calculation or the like. For example, the averagetemperature of the steel plate in the thickness direction may becalculated from a temperature distribution in the thickness directiondetermined by a finite difference method.

Steel Slab Heating Temperature: 1000° C. to 1200° C.

If the steel slab heating (reheating) temperature is less than 1000° C.,NbC does not dissolve sufficiently and, consequently, precipitationstrengthening may fail to be achieved in the subsequent step. Inaddition, the coarse undissolved carbides degrade HIC resistance. On theother hand, if the steel slab heating temperature is more than 1200° C.,the DWTT property becomes degraded. Accordingly, the steel slab heatingtemperature is limited to 1000° C. to 1200° C. The steel slab heatingtemperature is preferably 1000° C. or more and 1150° C. or less.

Cumulative Rolling Reduction Ratio in Non-Recrystallization TemperatureRange: 60% or More

In the step of hot-rolling the heated steel slab, after rolling has beenperformed in a recrystallization temperature range, rolling is performedin a non-recrystallization temperature range. The conditions under whichrolling is performed in the recrystallization temperature range are notlimited. For achieving high base metal toughness, it is necessary toperform sufficient rolling reduction within the non-recrystallizationtemperature range in the hot rolling process. However, if the cumulativerolling reduction ratio in the non-recrystallization temperature rangeis less than 60%, the size of crystal grains may fail to be reduced to asufficient degree. Consequently, a sufficient DWTT property may fail tobe achieved. Accordingly, the cumulative rolling reduction ratio in thenon-recrystallization temperature range is limited to 60% or more. Thecumulative rolling reduction ratio in the non-recrystallizationtemperature range is preferably 63% or more.

Finish Rolling Temperature: Ar₃ Transformation temperature or More and(Ar₃ Transformation temperature+60° C.) or Less

For limiting the reduction in strength due to the Bauschinger effect, itis necessary to form a metallic microstructure composed primarily ofbainite and suppress the formation of soft microstructures, such aspolygonal ferrite. This requires the hot rolling to be performed withinthe temperature range of the Ar₃ transformation temperature or more, inwhich polygonal ferrite does not form. Accordingly, the finish rollingtemperature is limited to be equal to or greater than the Ar₃transformation temperature and is preferably equal to or greater than(Ar₃ transformation temperature +10° C.). For achieving high base metaltoughness, it is necessary to perform the rolling at lower temperaturesin the temperature range of the Ar₃ transformation temperature or more.Accordingly, the upper limit for the finish rolling temperature is setto (Ar₃ transformation temperature +60° C.) The finish rollingtemperature is preferably equal to or less than (Ar₃ transformationtemperature +50° C.)

The Ar₃ transformation temperature can be calculated using Formula (3)below.

Ar₃(° C.)=910−310C−80Mn−20Cu−15Cr−55Ni−80Mo   (3)

Cooling Start Temperature: Ar₃ Transformation temperature or More

If the cooling start temperature is less than the Ar₃ transformationtemperature, the area fraction of polygonal ferrite may exceed 10% and asufficiently high compressive strength may fail to be achieved due to areduction in strength caused by the Bauschinger effect. Accordingly, thecooling start temperature is limited to be equal to or greater than theAr₃ transformation temperature. The cooling start temperature ispreferably equal to or greater than (Ar₃ transformation temperature +10°C.)

Cooling Rate: 10° C./s or More

Accelerated cooling performed at a cooling rate of 10° C./s or more is aprocess essential for producing a high strength steel plate having hightoughness. Performing cooling at a high cooling rate enables strength tobe increased due to transformation strengthening. However, if thecooling rate is less than 10° C./s, a sufficiently high strength mayfail to be achieved. Furthermore, diffusion of C may occur. This resultsin thickening of C at non-transformed austenite and an increase in theamount of MA formed. Consequently, compressive strength may be reduced,because the presence of hard second phases, such as MA, accelerates theBauschinger effect as described above. When the cooling rate is 10° C./sor more, diffusion of C which occurs during the cooling is suppressedand, consequently, the formation of MA is reduced. Accordingly, thecooling rate in the accelerated cooling is limited to 10° C./s or more.The cooling rate is preferably 20° C./s or more. If the cooling rate isexcessively high, hard microstructures, such as martensite, may beformed and, consequently, toughness may become degraded. In addition,compressive strength may be reduced due to the acceleration of theBauschinger effect. Accordingly, the cooling rate is preferably 200°C./s or less.

Cooling Stop Temperature: 200° C. to 450° C.

Performing rapid cooling to a temperature of 200° C. to 450° C. by theaccelerated cooling subsequent to the rolling enables the formation ofbainite phase and a uniform microstructure. However, if the cooling stoptemperature is less than 200° C., an excessive amount ofmartensite-austenite constituent (MA) may be formed. This results in areduction in compressive strength due to the Bauschinger effect anddegradation of toughness. On the other hand, if the cooling stoptemperature is more than 450° C., pearlite may be formed. This makes itnot possible to achieve a sufficiently high strength and results in areduction in compressive strength due to the Bauschinger effect.Accordingly, the cooling stop temperature is limited to 200° C. to 450°C. The cooling stop temperature is preferably 250° C. or more and ispreferably 430° C. or less.

Reheating Temperature: 350° C. or More at ⅛-Plate Thickness Position and530° C. or Less at Surface of Steel Material

Reheating is performed subsequent to the accelerated cooling. In theaccelerated cooling of the steel plate, the cooling rate in thesurface-layer portion of the steel plate is high, and the surface-layerportion of the steel plate is cooled to a lower temperature than theinside of the steel plate. Consequently, the martensite-austeniteconstituent (MA) is likely to be formed in the surface-layer portion ofthe steel plate. Since hard phases, such as MA, accelerate theBauschinger effect, heating the surface-layer portion of the steel platesubsequent to the accelerated cooling to decompose MA enables tosuppress the reduction in compressive strength due to the Bauschingereffect. If the reheating temperature is less than 350° C. at the ⅛-platethickness position, MA fails to be decomposed to a sufficient degree. Ifthe reheating temperature is more than 530° C. at the surface of thesteel material, strength may be reduced. This makes it difficult toachieve the predetermined strength. Since collapse resistant performanceis in correlation with the compressive strength of a portion of thesteel material which extends from the surface of the steel material tothe ⅛-plate thickness position, controlling the reheating temperature ofthe portion of the steel material which extends from the surface of thesteel material to the ⅛-plate thickness position enables the certainstrength to be maintained while decomposing MA. Accordingly, thereheating temperature is limited to 350° C. or more at the ⅛-platethickness position and 530° C. or less at the surface of the steelmaterial. The reheating temperature is preferably 370° C. or more at the⅛-plate thickness position and 520° C. or less at the surface of thesteel material.

Examples of means for performing reheating subsequent to the acceleratedcooling include, but are not limited to, atmosphere furnace heating, gascombustion, and induction heating. Induction heating is preferable inconsideration of economy, controllability, etc.

4. Method for Producing Line Pipe

A steel pipe (line pipe) can be produced using the steel plate (steelmaterial) according to aspects of the present invention or a steel plate(steel material) produced by the above-described method. Examples of amethod for forming the steel material into shape include a method inwhich a steel material is formed into the shape of a steel pipe by coldforming, such as a UOE process or press bending (also referred to as“bending press”). In the UOE process, the edges of a steel plate (steelmaterial) in the width direction are subjected to groove cutting edgepreparation and then crimped using a press machine. Subsequently, thesteel plate is formed into a cylindrical shape such that the edges ofthe steel plate in the width direction face each other using a U-ingpress machine and an O-ing press machine. Then, the edges of the steelplate in the width direction are brought into abutment with and weldedto each other. This welding is referred to as “seam welding”. The seamwelding is preferably performed using a method including two steps, thatis, a tack welding step of holding the cylindrical steel plate, bringingthe edges of the steel plate in the width direction into abutment witheach other, and performing tack welding; and a final welding step ofsubjecting the inner and outer surfaces of the seam of the steel plateto welding using a submerged arc welding method. After the seam welding,pipe expansion is performed in order to remove welding residual stressand to enhance the roundness of the steel pipe. In the pipe expansionstep, the expansion ratio (the ratio of a change in the outer diameterof the pipe which occurs during the pipe expansion to the outer diameterof the pipe before the pipe expansion) is set to 1.2% or less. This isbecause, if the expansion ratio is excessively high, compressivestrength will be significantly reduced due to the Bauschinger effect.The expansion ratio is preferably 1.0% or less. The expansion ratio ispreferably 0.4% or more and is more preferably 0.6% or more in order toreduce welding residual stress and enhance the roundness of the steelpipe.

Subsequent to the pipe expansion, a coating treatment may be performedin order to prevent corrosion. In the coating treatment, for example,the steel pipe that has been subjected to the pipe expansion is heatedto 200° C. or more and, subsequently, a resin known in the related artor the like is applied to the outer or inner surface of the steel pipe.

In the press bending, the steel plate is repeatedly subjected tothree-point bending to gradually change its shape and, thereby, a steelpipe having a substantially circular cross section is produced. Then,seam welding is performed as in the UOE process described above. Also inthe press bending, pipe expansion may be performed after the seamwelding.

5. Steel Material for Line Pipes

A steel material for line pipes according to aspects of the presentinvention has the above-described composition and the above-describedmetallic microstructure. The 0.23% compressive strength of a portion ofthe steel material, the portion extending from the surface of the steelmaterial to the ⅛-plate thickness position, in the transverse direction(rolling orthogonal direction) is 340 MPa or more. The temperature atwhich a percent ductile fracture of the steel material measured by aDWTT test becomes 85% or more is −10° C. or less. Since the 0.23%compressive strength of the portion of the steel material for line pipesaccording to aspects of the present invention which extends from thesurface to the ⅛-plate thickness position in the transverse direction is340 MPa or more, the steel material for line pipes has excellentcollapse resistant performance. Note that 0.23% compressive strength canbe determined by the method described in Examples below.

6. Line Pipe

A line pipe according to aspects of the present invention has theabove-described composition and the above-described metalmicrostructure. In addition, the 0.23% compressive strength of a portionof the line pipe, the portion extending from the inner surface of thepipe to the ⅛-wall thickness position, in the circumferential directionat the major axis position of the pipe is 340 MPa or more. The collapsepressure of the line pipe is 35 MPa or more. The temperature at which apercent ductile fracture of the line pipe measured by a DWTT testbecomes 85% or more is −10° C. or less. Since the 0.23% compressivestrength of the portion of the line pipe according to aspects of thepresent invention which extends from the inner surface to the ⅛-wallthickness position in the circumferential direction at the major axisposition is 340 MPa or more and the collapse pressure of the line pipeis 35 MPa or more, the line pipe has excellent collapse resistantperformance. The 0.23% compressive strength of a portion of the linepipe according to aspects of the present invention which has theabove-described composition and the above-described microstructure andincludes the coating layer formed by the coating treatment, the portionextending from the inner surface of the pipe to the ⅛-wall thicknessposition, in the circumferential direction at the major axis position is390 MPa or more. The collapse pressure of the line pipe is 40 MPa ormore. Thus, the line pipe has excellent collapse resistant performance.Note that the term “major axis position of pipe” used herein refers to,when considering a position in the circumferential direction of thepipe, a position 90 degrees from the position at which the radius of thepipe is the minimum. Note that 0.23% compressive strength can bedetermined by the method described in Examples below.

EXAMPLES

Slabs were manufactured from steels (Steel types A to J) having thechemical compositions described in Table 1 by a continuous castingprocess. The slabs were heated, hot-rolled, and then immediatelyprocessed in a water-cooling-type cooling equipment to performaccelerated cooling. Subsequently, reheating was performed using aninduction heating furnace or a gas combustion furnace. Hereby, steelplates (Nos. 1 to 23) having a thickness of 40 mm were prepared. Notethat the heating temperature, the finish rolling temperature, thecooling start temperature, and the cooling stop temperature weredetermined as the average temperatures over the steel plate, while thereheating temperature was measured at the surface and the ⅛-platethickness position. The average temperatures and the temperature at the⅛-plate thickness position were calculated on the basis of the surfacetemperature of the slab or steel plate using parameters such as platethickness, thermal conductivity, etc.

Pipes having a wall thickness of 39 mm and an outside diameter of 813 mmwere prepared using the above steel plates by an UOE process. Seamwelding was performed by four-wire submerged arc welding such that onewelding path was formed each on the inner and outer surfaces of thepipe. The heat input during the welding was selected from the range of100 kJ/cm in accordance with the thickness of the steel plate. Afterwelding, the pipes were expanded at an expansion ratio of 0.6% to 1.5%.The expanded pipes were further subjected to a coating treatment at 230°C. Table 2 summarizes the conditions under which the steel plates wereproduced and the condition under which the steel pipes were produced(expansion ratio).

TABLE 1 Ar₃ transformation Steel Composition (mass %) Ceq Pcmtemperature type C Si Mn Nb Ti Al Ca Cu Ni Cr Mo V value value (° C.)Remark A 0.055 0.050 1.82 0.021 0.013 0.033 0.280 0.040 0.422 0.164 740Invention B 0.042 0.100 1.75 0.023 0.020 0.028 0.210 0.190 0.360 0.147742 example C 0.093 0.080 1.58 0.023 0.011 0.031 0.100 0.080 0.070 0.3820.186 743 D 0.055 0.090 1.86 0.027 0.015 0.030 0.140 0.150 0.210 0.4260.175 716 E 0.053 0.050 1.93 0.025 0.011 0.030 0.180 0.020 0.415 0.165725 F 0.056 0.070 1.85 0.024 0.014 0.027 0.0023 0.100 0.200 0.240 0.4320.171 728 G 0.026 0.080 1.65 0.029 0.014 0.028 0.130 0.140 0.050 0.0050.330 0.124 756 Comparative H 0.078 0.070 1.80 0.022 0.011 0.032 0.1000.100 0.310 0.180 0.489 0.205 715 example I 0.110 0.060 1.62 0.026 0.0200.033 0.050 0.390 0.196 742 J 0.056 0.180 1.83 0.024 0.015 0.028 0.3100.060 0.010 0.437 0.174 737 *The underlined values are outside the scopeof the present invention. Ceq value = C + Mn/6 + (Cu + Ni)/15 + (Cr +Mo + V)/5 Pcm value = C + Si/30 + (Mn + Cu + Cr)/20 + Ni/60 + Mo/15 +V/10 Ar₃ transformation temperature = 910 − 310C − 80Mn − 20Cu − 15Cr −55Ni − 80Mo (The symbols of elements represent the content (mass %) ofthe elements.)

TABLE 2 Cumulative rolling reduction ratio in non- Finish CoolingHeating recrystallization rolling start Cooling Steel Thicknesstemperature temperature temperature temperature rate No. type (mm) (°C.) range (%) (° C.) (° C.) (° C./s) 1 A 40 1030 75 770 765 24 2 A 401080 80 785 780 22 3 A 40 1040 75 770 770 23 4 A 40 1050 75 765 760 25 5B 40 1050 75 780 770 35 6 C 40 1040 75 765 760 31 7 D 40 1070 75 765 76028 8 E 40 1100 75 760 760 26 9 F 40 1060 75 770 765 27 10 A 40  950 75770 765 22 11 A 40 1260 75 760 760 24 12 A 40 1040 55 765 760 22 13 A 401030 75 745 715 19 14 A 40 1030 75 805 800 26 15 D 40 1060 75 770 770 2016 D 40 1070 75 765 760 25 17 D 40 1070 75 765 760 27 18 A 40 1040 75765 760 22 19 D 40 1060 75 760 755 24 20 G 40 1050 75 775 770 26 21 H 401060 75 760 760 23 22 I 40 1030 75 760 755 21 23 J 40 1040 75 765 760 27Cooling Reheating stop temperature (° C.) temperature 1/8 Expansion No.(° C.) Reheating equipment Surface Position ratio (%) Remark 1 320Induction heating furnace 450 405 0.8 Invention 2 360 Induction heatingfurnace 475 430 0.8 example 3 310 Induction heating furnace 410 390 0.84 260 Induction heating furnace 515 430 0.8 5 330 Gas combustion furnace455 435 0.8 6 280 Induction heating furnace 460 395 0.8 7 310 Inductionheating furnace 450 400 1.0 8 390 Induction heating furnace 460 433 0.69 320 Induction heating furnace 455 405 1.0 10 310 Induction heatingfurnace 460 405 0.8 Comparative 11 330 Induction heating furnace 460 4100.8 example 12 320 Induction heating furnace 460 410 0.8 13 290Induction heating furnace 460 405 0.8 14 330 Induction heating furnace450 435 0.8 15 500 Induction heating furnace 510 500 0.8 16 330Induction heating furnace 380 335 0.8 17 270 Induction heating furnace570 500 0.8 18 340 None 0.8 19 320 Induction heating furnace 460 410 1.520 320 Induction heating furnace 460 410 0.8 21 330 Induction heatingfurnace 470 415 0.8 22 300 Induction heating furnace 480 410 0.8 23 310Induction heating furnace 470 405 0.8 *The underlined values are outsidethe scope of the present invention.

The compressive property of each of the steel plates produced in theabove-described manner was determined using a test piece for compressivetest that was taken from a portion of the steel plate extending from thesurface of the steel plate to the ⅛-plate thickness position.Specifically, a small piece of the steel plate for test piece forcompressive test was taken from the steel plate such that thelongitudinal direction of the small piece was equal to the transversedirection of the steel plate. The other surface of the small piece ofthe steel plate was cut or ground to reduce the thickness of the smallpiece to ⅛ of the plate thickness. Subsequently, a rectangular testpiece including a parallel part having a thickness of 2.5 mm, a width of2.5 mm, and a length of 4.0 mm was taken from the small piece of thesteel plate. In order to simulate pipe production, a compressive strainof 2.5% was applied to the test piece and a tensile strain of 1.0% wasthen applied to the test piece. The test piece that had been subjectedto the simulated pipe production was subjected to a compressive test inwhich a load was applied to the test piece in a compression direction.The stress at which the compressive strain was 0.23% in the resultingcompressive stress-strain curve was determined as a 0.23% compressivestrength.

The tensile property of each of the pipes produced in theabove-described manner was evaluated on the basis of the tensilestrength determined by a tensile test of a test piece that was acircumferential direction full-thickness test piece in accordance withAPI 5L. The compressive property of each of the pipes was determinedusing a test piece taken from the inner surface of the pipe at the majoraxis position of the pipe in the circumferential direction.Specifically, a piece of the pipe for test piece for compressive testwas taken from the pipe such that the longitudinal direction of thepiece was equal to the circumferential direction of the pipe. The pieceof the steel plate was cut or ground from the outer surface-side of thepipe to reduce the thickness of the piece to ⅛ of the plate thickness.Subsequently, a rectangular test piece including a parallel part havinga thickness of 2.5 mm, a width of 2.5 mm, and a length of 4.0 mm wastaken from the piece of the steel plate. The test piece was subjected toa compressive test in which a load was applied to the test piece in acompression direction. The stress at which the compressive strain was0.23% in the resulting compressive stress-strain curve was determined asa 0.23% compressive strength. In the measurement of collapse resistantperformance, each of the pipes was cut to 7 m and a water pressure wasgradually applied to the pipe inside a pressure vessel. The pressure atwhich the water pressure started decreasing was determined as a collapsepressure. Note that, the compression performance and collapse resistantperformance were determined both after pipe expansion (as formed) andafter the coating treatment at 230° C. (after heated at 230° C.)

A DWTT test piece was taken from each of the steel pipes in thecircumferential direction of the pipe. Using the DWTT test piece, thetemperature at which the percent ductile fracture became 85% wasdetermined as 85% SATT.

For determining the HAZ toughness of the joint, the temperature at whichthe percent ductile fracture was 50% was determined as vTrs. Theposition of the notch was determined such that the fusion line waslocated at the center of the notch root of the Charpy test piece and theratio between the weld metal and the base metal (including weldheat-affected zone) at the notch root was 1:1.

For determining metallic microstructure, a sample was taken from theinner surface-side portion of each of the steel pipes at the ⅛-platethickness position. A cross section of the sample which was parallel tothe longitudinal direction of the pipe was etched using nital afterpolishing, and the metallic microstructure was observed using an opticalmicroscope. The area fractions of bainite and polygonal ferrite werecalculated by image analysis of 3 photographs captured at a 200-foldmagnification. For observing MA, the sample used for measuring the areafractions of bainite and polygonal ferrite was subjected to nitaletching and then electrolytic etching (two-step etching). Subsequently,the metallic microstructure was observed with a scanning electronmicroscope (SEM). The area fraction of MA was calculated by imageanalysis of 3 photographs captured at a 1000-fold magnification.

Although the metallic microstructures of the pipes are determined inExamples, the results may be considered as the metallic microstructuresof the respective steel plates.

Table 3 shows the metallic microstructures and mechanical propertiesmeasured.

TABLE 3 Mechanical properties Metallic microstructure Pipe Area 0.23%Area fraction of Steel plate Compressive fraction martensite- 0.23%strength Area of austenite Compressive (MPa) Steel fraction of polygonalconstituent strength As No. type bainite (%) ferrite (%) (%) Balance(MPa) formed 1 A 95.8  3.0 0.8 θ 361 371 2 A 98.5  0.0 1.2 θ 378 382 3 A95.4  0.0 4.2 θ 347 355 4 A 94.8  3.5 0.3 θ 352 368 5 B 96.5  2.2 0.6 θ361 368 6 C 89.0  7.0 3.4 θ 363 375 7 D 98.3  0.0 1.3 θ 381 379 8 E 98.3 0.0 0.6 θ 369 382 9 F 98.5  0.0 1.5 — 382 383 10 A 94.4  4.5 0.7 θ 320326 11 A 95.0  3.6 1.3 θ 419 432 12 A 93.9  4.2 1.2 θ 351 356 13 A 67.326.0 6.2 θ 283 288 14 A 98.2  0.0 1.1 θ 369 378 15 D 94.9  0.0 0.4 θ, P301 309 16 D 93.2  0.0 6.8 — 304 311 17 D 98.0  0.0 0.3 θ 322 331 18 A87.8  5.1 7.1 — 317 320 19 D 98.1  0.0 1.1 θ 353 328 20 G 91.2  8.0 0.1θ 300 302 21 H 98.4  0.0 1.2 θ 401 412 22 I 83.9  7.5 7.2 θ, P 295 29623 J 89.4  4.8 5.8 — 316 328 Mechanical properties Pipe 0.23%Compressive strength Collapse pressure (MPa) (MPa) DWTT After Afterproperty HAZ heated heated Tensile 85% toughness at As at strength SATTvTrs No. 230° C. formed 230° C. (MPa) (° C.) (° C.) Remark 1 457 37.146.8 615 −27 −42 Invention 2 467 38.5 47.2 649 −20 −38 example 3 45336.7 46.1 635 −27 −40 4 446 36.3 46.1 597 −22 −38 5 453 36.9 46.4 578−26 −47 6 518 38.5 52.1 585 −33 −28 7 463 38.2 47.2 645 −25 −33 8 46239.0 46.8 643 −37 −44 9 465 38.4 47.3 648 −28 −35 10 424 33.3 43.2 559−40 −41 Comparative 11 550 44.0 51.9 741 −5  −40 example 12 463 36.547.2 614 −5  −38 13 555 29.1 51.1 573 −47 −42 14 461 37.6 47.1 641 −5 −41 15 373 31.2 38.2 552 −28 −35 16 417 31.0 42.8 663 −30 −35 17 39933.9 40.9 560 −28 −33 18 463 32.8 47.1 693 −27 −43 19 401 33.6 41.0 626−30 −32 20 415 31.4 42.5 521 −27 −52 21 503 43.2 48.9 699 −17 −5  22 45130.5 46.2 560 −25 −10 23 463 33.6 46.9 627 −23 −36 *The underlinedvalues are outside the scope of the present invention. * θ: Cementite,P: Pearlite

In Table 3, all of Nos. 1 to 9 had a tensile strength of 570 MPa ormore; 0.23% compressive strengths of 340 MPa or more as steel plate, 340MPa or more as formed, and 390 MPa or more after heated at 230° C.;collapse pressures of 35 MPa or more as formed and 40 MPa or more afterheated at 230° C.; as for a DWTT property, a 85% SATT of −10° C. orless; and a HAZ toughness of −20° C. or less. That is, all of Nos. 1 to9 were evaluated as good.

In contrast, in Nos. 10 to 19, although the composition fell within thescope according to aspects of the present invention, the productionmethod was outside the scope of the present invention and therefore theintended microstructure was not formed. As a result, Nos. 10 to 19 wereevaluated as poor in terms of any of tensile strength, 0.23% compressivestrength, and DWTT property. In Nos. 20 to 23, the chemical compositionwas outside the scope of the present invention. As a result, Nos. 20 to23 were evaluated as poor in terms of any of tensile strength,compressive strength, DWTT property, and HAZ toughness.

According to aspects of the present invention, a steel pipe of API X70grade or more which has a high strength and excellent low-temperaturetoughness may be produced. This steel pipe may be used as deep-oceanline pipes that require high collapse resistant performance.

1. A steel material for line pipes, the steel material comprising acomposition containing, by mass, C: 0.030% to 0.10%, Si: 0.01% to 0.15%,Mn: 1.0% to 2.0%, Nb: 0.005% to 0.050%, Ti: 0.005% to 0.025%, and Al:0.08% or less, the composition further containing one or more elementsselected from, by mass, Cu: 0.5% or less, Ni: 1.0% or less, Cr: 1.0% orless, Mo: 0.5% or less, and V: 0.1% or less, wherein a Ceq valuerepresented by Formula (1) is 0.35 or more and a Pcm value representedby Formula (2) is 0.20 or less, with the balance being Fe and incidentalimpurities, wherein a metallic microstructure of the steel material at a⅛-plate thickness position relative to a surface of the steel materialincludes bainite of an area fraction of 85% or more, polygonal ferriteof an area fraction of 10% or less, and martensite-austenite constituentof an area fraction of 5% or less, and wherein a 0.23% compressivestrength of a portion of the steel material, the portion extending fromthe surface of the steel material to the ⅛-plate thickness position, ina rolling orthogonal direction is 340 MPa or more, and a temperature atwhich a percent ductile fracture of the steel material measured in aDWTT test becomes 85% or more is −10° C. or less,Ceq value=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5   (1)Pcm value=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10   (2) where, inFormulae (1) and (2), the symbol of each element represents the content(mass%) of the element and is zero when the steel material does notcontain the element.
 2. The steel material for line pipes according toclaim 1, the steel material further comprising, by mass, Ca: 0.0005% to0.0035%.
 3. A method for producing a steel material for line pipes,wherein a 0.23% compressive strength of a portion of the steel material,the portion extending from a surface of the steel material to a ⅛-platethickness position, in a rolling orthogonal direction is 340 MPa ormore, and a temperature at which a percent ductile fracture of the steelmaterial measured in a DWTT test becomes 85% or more at −10° C. or less,the method comprising: heating a steel having the composition accordingto claim 1 to a temperature of 1000° C. to 1200° C.; hot-rolling thesteel such that a cumulative rolling reduction ratio in anon-recrystallization temperature range is 60% or more, and such that afinish rolling temperature is equal to or greater than an Ar₃transformation temperature and equal to or less than (Ar₃ transformationtemperature+60° C.); subsequently performing accelerated cooling from atemperature equal to or greater than the Ar₃ transformation temperatureto a temperature of 200° C. to 450° C. at a cooling rate of 10° C./s ormore; and then performing reheating such that a temperature of the steelmaterial at the ⅛-plate thickness position is 350° C. or more and suchthat a temperature of the surface of the steel material is 530° C. orless.
 4. A method for producing a steel material for line pipes, whereina 0.23% compressive strength of a portion of the steel material, theportion extending from a surface of the steel material to a ⅛-platethickness position, in a rolling orthogonal direction is 340 MPa ormore, and a temperature at which a percent ductile fracture of the steelmaterial measured in a DWTT test becomes 85% or more at −10° C. or less,the method comprising: heating a steel having the composition accordingto claim 2 to a temperature of 1000° C. to 1200° C.; hot-rolling thesteel such that a cumulative rolling reduction ratio in anon-recrystallization temperature range is 60% or more, and such that afinish rolling temperature is equal to or greater than an Ar₃transformation temperature and equal to or less than (Ar₃ transformationtemperature+60° C.); subsequently performing accelerated cooling from atemperature equal to or greater than the Ar₃ transformation temperatureto a temperature of 200° C. to 450° C. at a cooling rate of 10° C./s ormore; and then performing reheating such that a temperature of the steelmaterial at the ⅛-plate thickness position is 350° C. or more and suchthat a temperature of the surface of the steel material is 530° C. orless.
 5. A line pipe comprising a composition containing, by mass, C:0.030% to 0.10%, Si: 0.01% to 0.15%, Mn: 1.0% to 2.0%, Nb: 0.005% to0.050%, Ti: 0.005% to 0.025%, and Al: 0.08% or less, the compositionfurther containing one or more elements selected from, by mass, Cu: 0.5%or less, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 0.5% or less, and V:0.1% or less, wherein a Ceq value represented by Formula (1) is 0.35 ormore and a Pcm value represented by Formula (2) is 0.20 or less, withthe balance being Fe and incidental impurities, wherein a metallicmicrostructure of the line pipe at a ⅛-wall thickness position relativeto an inner surface of the line pipe, includes bainite of an areafraction of 85% or more, polygonal ferrite of an area fraction of 10% orless, and martensite-austenite constituent of an area fraction of 5% orless, and wherein a 0.23% compressive strength of a portion of the linepipe, the portion extending from the inner surface of the line pipe tothe ⅛-wall thickness position, in a circumferential direction of theline pipe at a major axis position of the line pipe is 340 MPa or more,a collapse pressure of the line pipe is 35 MPa or more, and atemperature at which a percent ductile fracture of the line pipemeasured in a DWTT test becomes 85% or more is −10° C. or less,Ceq value=C+Mn/6+(Cu+Ni)/15+(Cr+Mo+V)/5   (1)Pcm value=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10   (2) where, inFormulae (1) and (2), the symbol of each element represents the content(mass%) of the element and is zero when the line pipe does not containthe element.
 6. The line pipe according to claim 5, the line pipefurther comprising, by mass, Ca: 0.0005% to 0.0035%.
 7. The line pipeaccording to claim 5, the line pipe further comprising a coating layer.8. The line pipe according to claim 6, the line pipe further comprisinga coating layer.
 9. A method for producing a line pipe, wherein a 0.23%compressive strength of a portion of the line pipe, the portionextending from an inner surface of the line pipe to a ⅛-wall thicknessposition, in a circumferential direction of the line pipe at a majoraxis position of the line pipe is 340 MPa or more, a collapse pressureof the line pipe is 35 MPa or more, and a temperature at which a percentductile fracture of the line pipe measured in a DWTT test becomes 85% ormore is −10° C. or less, the method comprising cold forming the steelmaterial for line pipes according to claim 1 into a pipe-like shape;joining butting edges to each other by seam welding; and subsequentlyperforming pipe expansion at an expansion ratio of 1.2% or less toproduce a pipe.
 10. A method for producing a line pipe, wherein a 0.23%compressive strength of a portion of the line pipe, the portionextending from an inner surface of the line pipe to a ⅛-wall thicknessposition, in a circumferential direction of the line pipe at a majoraxis position of the line pipe is 340 MPa or more, a collapse pressureof the line pipe is 35 MPa or more, and a temperature at which a percentductile fracture of the line pipe measured in a DWTT test becomes 85% ormore is −10° C. or less, the method comprising cold forming the steelmaterial for line pipes according to claim 2 into a pipe-like shape;joining butting edges to each other by seam welding; and subsequentlyperforming pipe expansion at an expansion ratio of 1.2% or less toproduce a pipe.
 11. A method for producing a line pipe, wherein a 0.23%compressive strength of a portion of the line pipe, the portionextending from an inner surface of the line pipe to a ⅛-wall thicknessposition, in a circumferential direction of the line pipe at a majoraxis position of the line pipe is 340 MPa or more, a collapse pressureof the line pipe is 35 MPa or more, and a temperature at which a percentductile fracture of the line pipe measured in a DWTT test becomes 85% ormore is −10° C. or less, the method comprising cold forming a steelmaterial for line pipes produced by the method according to claim 3 intoa pipe-like shape; joining butting edges to each other by seam welding;and subsequently performing pipe expansion at an expansion ratio of 1.2%or less to produce a pipe.
 12. A method for producing a line pipe,wherein a 0.23% compressive strength of a portion of the line pipe, theportion extending from an inner surface of the line pipe to a ⅛-wallthickness position, in a circumferential direction of the line pipe at amajor axis position of the line pipe is 340 MPa or more, a collapsepressure of the line pipe is 35 MPa or more, and a temperature at whicha percent ductile fracture of the line pipe measured in a DWTT testbecomes 85% or more is −10° C. or less, the method comprising coldforming a steel material for line pipes produced by the method accordingto claim 4 into a pipe-like shape; joining butting edges to each otherby seam welding; and subsequently performing pipe expansion at anexpansion ratio of 1.2% or less to produce a pipe.
 13. The method forproducing a line pipe according to claim 9, the method furthercomprising performing a coating treatment subsequent to the pipeexpansion, the coating treatment including heating the pipe such that atemperature of the surface of the pipe reaches 200° C. or more.
 14. Themethod for producing a line pipe according to claim 10, the methodfurther comprising performing a coating treatment subsequent to the pipeexpansion, the coating treatment including heating the pipe such that atemperature of the surface of the pipe reaches 200° C. or more.
 15. Themethod for producing a line pipe according to claim 11, the methodfurther comprising performing a coating treatment subsequent to the pipeexpansion, the coating treatment including heating the pipe such that atemperature of the surface of the pipe reaches 200° C. or more.
 16. Themethod for producing a line pipe according to claim 12, the methodfurther comprising performing a coating treatment subsequent to the pipeexpansion, the coating treatment including heating the pipe such that atemperature of the surface of the pipe reaches 200° C. or more.